The E. coli exonuclease VII holoenzyme is composed of two subunits: ` is a 52 kD protein coded by the xseA gene and is a 9 kD protein coded by the xseB gene. Both subunits are required for enzymatic activity, but the holoenzyme molecular mass and the subunit stoichiometry is unknown. The xse genes have been cloned and cloned exo VII holoenzyme has been produced which has the same enzymatic properties as the native enzyme. The hope was to compare the masses of the cloned enzyme and the native enzyme (of which only small quantities are available). Preliminary studies were initiated with the cloned enzyme. Unfortunately, the specimen appeared quite heterogeneous. Further studies await additional purification.